Email recipients are often spoofed by email senders masquerading as a trustworthy source. Efforts have been made to verify the true identity of email senders to aid email recipients in knowing whether to trust the source of the email. Such efforts include implementing the S/MIME standard, per RFC5751, to digitally sign emails with public key certificates issued by certification authorities that enable a recipient to verify information about an email's integrity and information about the sender, such as the sender's name, which organization issued the sender's digital certificate, or the sender's email address. Digital signatures of email senders can be validated using public key certificates, but the degree of validation is not known to the email recipient because conventional validation protocols do not take into account the degree of identity proofing performed by the certification authority that issued the public key certificate. Accordingly, an email recipient may receive a binary indication (e.g., “trusted” or “not trusted”) as to the validity of a digital signature, but does not know the degree of trust that can be attributed to the digital signature. The present invention overcomes some of the problems inherent in conventional certificate validation.